There has been a need for improved light sources capable of generating ultraviolet light in the spectral region between about 50 and 200 nanometers wavelength, commonly referred to as the "vacuum ultraviolet" or "VUV" region. VUV photons have energies on the order of 10 electron volts (10 ev) and are capable of breaking chemical bonds of many compounds. Thus, VUV light can be used to accelerate chemical reactions as in light enhanced chemical vapor deposition curing of photosensitive materials, production of ozone and cracking gaseous waste products. Moreover, the minimum feature size which can be imaged with light is directly proportional to the wavelength of the light. VUV light has the shortest wavelength of any light which can be focused and reflected with conventional optical elements. Therefore, photographic processes employing VUV light can image smaller feature sizes than those imaginable with other light wavelengths. This is of particular importance in photographic processes used to fabricate semiconductors.
Additional needs exist for broadband VUV light sources, i.e., light sources which emit the VUV light over a continuum of wavelengths within the VUV range. A broadband source can be used for absorption spectroscopy in the VUV range. Because gases such as hydrogen and oxygen have resonance lines in this range, VUV absorption spectroscopy can be used for sensitive analytical measurements. A light source for use in spectroscopy desirably can operate continuously, with stable emission characteristics over time. A stable, continuously operable broadband VUV source is also needed for use as a calibration standard, for measuring the sensitivity of VUV light detection systems in laboratory apparatus.
No truly satisfactory broadband VUV light source has been available heretofore. Deuterium arc lamps have been used. However, such lamps emit a relatively weak continuum or broadband radiation in the VUV range together with intense line radiation at particular wavelengths. This spectral characteristic requires that the detector system used to measure the light have a very high dynamic range, i.e., the capability of measuring a weak light at some wavelengths and very intense light at others.
Some consideration has been given to the use of excimer radiation as a source of VUV light. Excimers are temporary chemical compounds composed of atoms which normally do not combine with one another. One or more of the atoms constituting an excimer is in an excited state, i.e.,a state in which the electrons have been momentarily promoted to a high-energy state. The excimer molecule as a whole is also in an excited state, and will ultimately decay to yield the constituent atoms. For example, elements commonly referred to as inert gases, helium, neon, argon, krypton and xenon, which normally exist only as isolated atoms can form excimer molecules when in the excited state. Diatomic inert gas excimers such as Ar.sub.2 *, Kr.sub.2 *, and Xe.sub.2 * emit broadband continuum radiation in the VUV range. However, to form these excimers in appreciable quantities, it is necessary to provide excitation energies on the order of 10 to 40 electron volts per atom. Moreover, this excitation energy must be provided while the gas is maintained at relatively low temperatures, typically below 200.degree. C. and most typically about room temperature. The gas also should be maintained under appreciable absolute pressure, desirably at least about 100 millibar (mbar) and most preferably about 0.5 bar or more, i.e., most preferably at about atmospheric pressure or more. Such substantial gas pressure is needed to provide a dense gas, which facilitates the excimer forming reactions. A simple direct current electrical arc discharge is ineffective to form excimers under these conditions. Other specialized arc discharge arrangements such as surface barrier discharges or arcs applied in short pulses cannot produce excimer light, these devices operate only intermittently and do not provide stable, continuous emission.
Excimers have been produced by applying beams of energetic particles, particularly electron beams, to the gas. However, the gas must be at substantial pressure as discussed above, whereas electron beams typically must be formed and focused in a high vacuum. Therefore, it is necessary to inject the electron beam into the gas to a thin region or "window" in the wall of the gas-containing chamber. Metal windows such as titanium foils have been used for electron beam windows heretofore. However, because these windows must withstand the gas pressure, they must have appreciable thickness, typically about two microns or more for a window diameter of about 5 millimeters. For efficient penetration of such a titanium foil, the electrons in the beam must have energies above 100 KeV. Electron accelerators capable of generating electron beams with these beam energies are bulky and expensive and impractical for use in a light source. Perhaps for these reasons, formation of excimers by injection of electron beams have been confined heretofore to basic laboratory research studies and has not found practical application.
Accordingly, there is still a significant, unmet need for improved broadband VUV light sources. There are other, corresponding needs for improved monochromatic VUV sources, particularly at the shorter-wavelength end of the VUV spectrum. Moreover, transient species including inert gas atoms and other atoms in excited metastable states, as well as highly reactive chemical species such as single atoms of normally diatomic gases can be used to facilitate chemical reactions. The transient species store energy which drives the chemical reactions. For example, low-pressure plasmas are commonly used to form reactive species such as monatomic hydrogen, which in turn is reacted with a workpiece such as a semiconductor wafer to etch the wafer or to strip photoresist therefrom. However, plasma processes generally operate at very low pressures and produce low densities of transient species, which limits the speed of the reaction. Thus, there are also needs for improved methods of making transient species.